Implantable wireless accoustic stimulators with high energy conversion efficiencies

ABSTRACT

Receiver-stimulator with folded or rolled up assembly of piezoelectric components, causing the receiver-stimulator to operate with a high degree of isotropy are disclosed. The receiver-stimulator comprises piezoelectric components, rectifier circuitry, and at least two stimulation electrodes. Isotropy allows the receiver-stimulator to be implanted with less concern regarding the orientation relative the transmitted acoustic field from an acoustic energy source.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/059,228 (Attorney Docket No. 41567-725.401, now U.S. Pat. No.______), filed Oct. 21, 2013, which is a divisional of U.S. patentapplication Ser. No. 13/734,680 (Attorney Docket No. 41567-725.502, nowU.S. Pat. No. 8,588,926), filed Jan. 4, 2013, which is acontinuation-in-part of U.S. patent application Ser. No. 12/721,483(Attorney Docket No. 41567-725.501, now U.S. Pat. No. 8,364,276), filedMar. 10, 2010, which is a continuation-in-part of International (PCT)Patent Application No. PCT/US2009/038258 (Attorney Docket No.41567-725.601), filed Mar. 25, 2009, which claimed the benefit of U.S.Provisional Patent Application No. 61/039,340 (Attorney Docket No.41567-725.101), filed on Mar. 25, 2008, the disclosures of which areincorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a wireless acoustic stimulation systemfor stimulating biological tissue and, in particular, for areceiver-stimulator that converts an acoustic field into electricalpower at high energy conversion efficiency to deliver stimulation energyto tissue. The receiver-stimulator has highly isotropic performancebased on the mechanical and electrical arrangement of multiple acousticpower harvesting elements in the receiver-stimulator unit.

2. Description of the Background Art

Stimulation of cardiac tissue using acoustic energy based systemscomprising a controller-transmitter and one or more implantedreceiver-stimulator devices has recently been proposed by the inventorsof this patent application and described in detail, for example inpublished US Application Publication No. 2006/0136004. Thecontroller-transmitter transmits acoustic energy by producing anacoustic field that is transmitted over time. The acoustic field is apropagating acoustic wave defined by its direction and its intensity(i.e., its power per unit area, typically expressed as Watts/meter²).The acoustic field varies and attenuates as it propagates through thebody due to absorption, refraction, and reflection. To minimize losses,the controller-transmitter focuses, or attempts to maximize, theacoustic field on the receiver-stimulator. In turn, thereceiver-stimulator maximizes harvesting and converting of the acousticfield impinging upon it into electrical power delivered over time to thetissue to stimulate the tissue (stimulation energy). In general, thisreceiver-stimulator is a specialized transducer, that is, a device thatconverts acoustic power to electrical power. In another perspective thereceiver-stimulator uses the converted power as a tissue stimulator thatdelivers electrical energy to cardiac or other tissue through tissuestimulation electrodes. The controller-transmitter may be appliedexternally on the body, but will usually be implanted in the body,requiring that the controller-transmitter have a reasonable size,similar to that of implantable pacemakers, and that thecontroller-transmitter be capable of operating from batteries for alengthy period, typically three or more years. The relatively small sizeand relatively long operational period make it desirable that thereceiver-stimulators harvest as much of the acoustic field transmittedby the controller-transmitter as possible. Furthermore, it is desirableto maximize the isotropy of the receiver-stimulator, whereby theelectrical output power delivered to the tissue is constant or nearlyconstant as the receiver-stimulator's orientation is varied relative tothe propagation direction of the acoustic field transmitted by thecontroller-transmitter, as this orientation is not always predictable.

Piezoelectric components, i.e., piezoelectric transducers, are typicallyused in acoustic applications to convert mechanical vibrations, such asin an acoustic field, to electrical power. They can also be used in thereverse to convert electrical power into a mechanical, vibrational wave,e.g., an acoustic wave. Coupling of mechanical vibrations in an acousticfield to piezoelectric transducers is an important consideration. Themechanical structure, or portions of the mechanical structuresurrounding a piezoelectric component, which is exposed to the acousticfield determines the aperture, or surface, for coupling the acousticfield into the piezoelectric component. Generally, there is a tradeoffbetween aperture size/isotropy and the electrical power produced by anassociated piezoelectric component. On the one hand a large aperture isdesired to collect more acoustic power (and can then convert it to moreelectrical power). However, this comes at the expense of isotropy. Thelarger an aperture relative to the wavelength of the acoustic field itis placed in, the less isotropic it becomes. Therefore, areceiver-stimulator consisting of a single piezoelectric component islimited in its ability to produce high electrical output and exhibithigh isotropy. It can either produce high electrical output power orhigh isotropy, but not both.

The piezoelectric components produce AC electrical power which is notoptimized for tissue stimulation. A rectifier component is used toconvert this electrical power to an electrical output which can beconfigured to effectively stimulate the tissue (e.g., into a DC outputbut other output waveforms are also effective). Furthermore, the ACelectrical power produced by separate piezoelectric components can beout of phase, making it difficult to combine these outputs directlywithout loss of power. Rectifying these outputs prior to combining themreduces this power loss. Therefore, it would be advantageous to have onerectifier associated with each piezoelectric component. Furthermore, wecan view the combination of a piezoelectric component, its associatedaperture and rectifier as a single harvesting element that is capable ofproducing electrical power when placed in an acoustic field. Thereceiver-stimulator is then a collection of multiple harvesting elementswhose electrical outputs are combined to deliver an electrical output totissue in order to stimulate the tissue.

Once constructed, it is important to consider the assessment of theefficiency and isotropy of the entire receiver-stimulator rather thanthe individual harvesting elements. The Effective Area of thereceiver-stimulator can be defined in terms of the electrical outputpower delivered to the tissue divided by the acoustic intensity(power/(power/meter²)). Efficiency is then a measure of the EffectiveArea divided by the physical cross-sectional area of thereceiver-stimulator that is exposed to an acoustic field. The HighestEfficiency then would be when the Effective Area approximates thephysical cross-sectional area of the receiver-stimulator. To associateefficiency with isotropy, the aggregate performance over all possibleorientations of the receiver-stimulator to the acoustic field must beconsidered. It would be desirable to have a receiver-stimulator that hashigh efficiency and a high degree of isotropy.

It would be desirable to provide implantable receiver-stimulator deviceswhich are able to efficiently harvest power from an acoustic fieldtransmitted from implanted or external acoustic transmitters and convertthe acoustic power into stimulating electrical energy in an efficientmanner. It would be particularly desirable if the receiver-stimulatorscould operate with a high degree of isotropy, where the electricaloutput power delivered to the tissue is constant or nearly constant asthe receiver-stimulator's orientation is varied relative to thepropagation direction of the acoustic field transmitted by thecontroller-transmitter, irrespective of whether the individualharvesting elements in the receiver-stimulator are themselves consideredto be isotropic or non-isotropic. At least some of these objectives willbe met by the inventions described hereinafter.

The following patents and patent publications describe variousimplantable transducers capable of converting applied acoustic wavesinto an electrical output: U.S. Pat. Nos. 3,659,615; 3,735,756;5,193,539; 6,140,740; 6,504,286; 6,654,638; 6,628,989; and 6,764,446;U.S. patent application Publications 2002/0077673; 2004/0172083; and2004/0204744; and published German application DE 4330680. U.S. Pat. No.6,504,286 by Porat et al. describes a miniature piezoelectric transducerfor providing maximal electric output when impinged by external acousticwaves in the low frequency range. The patent discloses varioustechniques including the aggregate mechanical structure of the devicebeing omni-directional, changing the mechanical impedance of thepiezoelectric layer, etc. As mentioned earlier, it would be desirable tohave the receiver-stimulator itself be virtually isotropic rather thanrely on the isotropy characteristics of the individual harvestingelements.

BRIEF SUMMARY OF THE INVENTION

Systems and methods are provided for delivering electrical energy tobody tissues for many therapeutic indications. The electrical energywill typically be delivered in order to stimulate tissue, for example tostimulate cardiac tissue for therapeutically pacing the heart to treatbradycardia, for termination of tachyarrhythmia, for bi-ventricularresynchronization therapy for heart failure, or the like. The systemsand methods of the present invention, however, could be used in avariety of other applications, including applications for nervestimulation, brain stimulation, voluntary muscle stimulation, gastricstimulation, bone growth stimulation, pain amelioration, and the like.

In a first aspect, the present invention provides an implantablereceiver-stimulator device which is capable of harvesting acoustic powerfrom an acoustic field that is delivered from an acoustic source(physically separate from the receiver-stimulator device) and convertingthat acoustic power to electrical power to deliver electrical energy tostimulate tissue. The receiver-stimulator of the present invention isconfigured to be efficient, meaning it harvests all or nearly all of theacoustic power available and converts it into electrical powersufficient to stimulate cardiac tissue. In addition to efficientharvesting of acoustic power, the implantable receiver-stimulators ofthe present invention are also capable of functioning with a high degreeof isotropy. This means that the output electrical power of thereceiver-stimulator is constant or nearly constant as thereceiver-stimulator's orientation is varied, relative to the propagationdirection of the acoustic field transmitted by thecontroller-transmitter. In addition, the receiver-stimulator consists ofmultiple harvesting elements integrated into a mechanical structure thatin aggregate provide high efficiency and a high degree of isotropy.

In a first specific embodiment, an implantable receiver-stimulatorcomprises a hermetically sealed enclosure with an inner and outersurface and a plurality of harvesting elements. Each harvesting elementconsists of a piezoelectric component with one face of the componentaffixed to the inner surface defining an aperture for coupling anacoustic field from the outer surface of the receiver-stimulator to thepiezoelectric component and a rectifier circuit electrically connectedto the output of the piezoelectric component that produces an electricalsignal from each harvesting element. A mechanism is provided forcombining the output of multiple harvesting elements to produce abiologically stimulating electrical output, such as output suitable forcardiac pacing, nerve stimulation, brain stimulation, voluntary musclestimulation, pain amelioration, or the like. At least two electrodes inelectrical contact with the tissue are coupled to the rectifiercircuitry to receive the stimulating electrical output and deliver saidoutput to tissue. Either or both of the stimulation electrodes may bemounted directly on the device, in some instances forming a portion ofthe device casing, or extend from the device.

One embodiment of the receiver-stimulator device is a hermeticallysealed structure with an octagonal cross-section, optionally with an endcap at the proximal end of the longitudinal axis and another at thedistal end. The device may also be shaped substantially cylindrically.The device has an inner and outer surface and is constructed using anelectrically conductive base that is preferably biocompatible, such astitanium, upon which a circuit layer is built on the inner surface ofthe device. Additionally, a plurality of harvesting elements areorganized on the inner surface of the device, creating multipleapertures for harvesting acoustic power from the outer surface of thereceiver-stimulator. Each harvesting element consists of a piezoelectriccomponent attached to its associated aperture and electric connectionsthat are connected to its associated rectifier, with the piezoelectricelement and the rectifier both mounted to the inner surface. The innersurface may also contain additional circuitry that combines the outputpower from each of the individual harvesting elements to the pair ofelectrodes that are in electrical contact with the tissue. Theelectrodes are located on the exterior surface of, or otherwise attachedto or extended frorm, the receiver-stimulator device.

The proximal end of the receiver-stimulator can have mechanical orelectromechanical arrangements to engage and disengage the receiver froma placement catheter. The distal end of the device has a retractabletissue engagement mechanism that enables the receiver-stimulator to beattached to a desired location at the treatment site. One of thestimulation electrodes may be part of the tissue engagement mechanismand the other stimulation electrode may be located on the exteriorsurface of the receiver-stimulator.

In another aspect of the present invention, methods for deliveringacoustic power to an implanted receiver-stimulator comprise implanting areceiver-stimulator, typically formed as an assembly containing multipleharvesting elements, the receiver-stimulator having a high degree ofisotropy as described above in connection with the devices of thepresent invention. An acoustic field is directed to the implantedreceiver-stimulator assembly from an acoustic source, which may beimplanted or located externally, and the receiver-stimulator outputselectrical power to tissue in proportion to the acoustic intensityimpinging on the receiver-stimulator such that the minimum effectivearea is no more than −3 dB from the maximum effective area as theorientation of the receiver-stimulator varies relative to that of theacoustic source. With sufficient acoustic intensity the harvestedacoustic power is converted to electrical power and is rectified bycircuitry to produce a biologically stimulating electrical output, andthe electrical output is delivered to tissue. The acoustic field may bedelivered to the receiver-stimulator from an external source, but willpreferably be delivered from an implanted acoustic source. Theelectrical output flowing over time between stimulation electrodes whichare in electrical contact with tissue may possess specificcharacteristics of voltage, current, waveform, and the like. Theseelectrical characteristics will be selected to stimulate the targetcardiac tissue, nerve tissue, brain tissue, voluntary muscle tissue,bone tissue, or the like.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention has other advantages and features which will be morereadily apparent from the following detailed description of theinvention and the appended claims, when taken in conjunction with theaccompanying drawings, in which:

FIG. 1A shows a receiver-stimulator with the anchoring mechanismretracted.

FIG. 1B shows the receiver-stimulator with the anchoring mechanismdeployed.

FIG. 1C shows the receiver-stimulator cross-section in the retractedstate.

FIG. 1D shows the receiver-stimulator cross-section in the deployedstate.

FIG. 2 is a flow diagram illustrating a method for manufacturing areceiver-stimulator.

FIG. 3A shows a can assembly with piezoelectric components andrectifiers organized in an exemplary arrangement.

FIG. 3B is a side-view of the can assembly and exemplary piezoelectriccomponents.

FIG. 3C shows a folded can assembly.

FIG. 3D is a cross-sectional view of the folded can assembly.

FIGS. 3E-3I show an alternative can assembly with piezoelectriccomponents and rectifiers organized in an exemplary arrangement.

FIG. 4A shows a partial receiver-stimulator assembly with two end capsand an axle assembly.

FIG. 4B shows a polyimide, polyester, or liquid crystal polymer (LCP)flex circuit for the end cap.

FIG. 4C shows an end cap assembly.

FIG. 4D is a cross-sectional view of an end cap assembly.

FIG. 5A is a flow diagram illustrating a method for assembling an endcap.

FIG. 5B is a flow diagram illustrating a method for assembling an axleassembly.

FIG. 6A shows an electrical circuit with harvesting elements connectedin parallel.

FIG. 6B shows an electrical circuit with harvesting elements connectedin series.

DETAILED DESCRIPTION OF THE INVENTION

In the following description, for purposes of explanation, numerousspecific details are set forth in order to provide a thoroughunderstanding of the invention. It will be apparent, however, to oneskilled in the art that the invention can be practiced without thesespecific details.

In a first aspect, the present invention provides an implantablereceiver-stimulator (hereinafter also abbreviated as “R-S” and alsoreferred to as “stimulator”) device which is capable of wirelesslyharvesting acoustic power from an acoustic field delivered from anacoustic source physically separate from the R-S, and converting thatacoustic power to electrical power, rectifying the electrical power, anddelivering an electrical output between two stimulation electrodes.

FIGS. 1A-D are diagrams illustrating an implantable receiver-stimulator101, constructed in accordance with the principles of the presentembodiments. FIGS. 1C and ID are cross-sectional views of the R-S 101device as shown in FIGS. 1A and 1B, respectively.

The R-S 101 comprises an axle assembly 104 containing amoveable/retractable needle assembly 107 with an anchoring mechanism 102at its distal end and a detachment mechanism 103 at its proximal end.The anchoring mechanism 102 enables the R-S 101 to be attached to adesired location at the treatment site. The axle assembly 104 comprisesan internal cathode feed-through 106 to the needle assembly 107. Theneedle assembly 107 is configured to move axially in R-S 101 through theaxle assembly 104. The proximal end of the R-S 101 may comprisemechanical or electro-mechanical arrangements to engage and disengagethe R-S 101 from a delivery catheter.

The needle assembly 107, including the anchoring and detachmentmechanisms 102 and 103, is configured to start out in a retracted stateas shown in FIGS. 1A and 1C. When the R-S 101 is to be permanentlyattached to patient tissue, the needle assembly 107 assumes a deployedstate as the detachment mechanism 103 is pushed axially forward, forexample by a delivery catheter. As shown in FIGS. 1B and 1D, pushing theneedle assembly 107 axially forward will insert the anchoring mechanism102 into patient tissue (delivery catheter not shown).

In the retracted state shown in FIGS. 1A and 1C, the anchoring mechanism102 is retracted inside the R-S 101 with one or more barbs 105 held inplace along the tip of the needle assembly 107, and the detachmentmechanism 103 is extended outwards at the proximal end of the R-S 101.

In the deployed state shown in FIGS. 1B and 1D, the anchoring mechanism102 is pushed distally outwards, allowing its barbs 105 to fan outradially and thereby attach the R-S 101 to patient tissue.

As shown in FIGS. 1C and 1D, the R-S 101 comprises a hermetically sealedenclosure 110. The enclosure 110 has an outer surface 111 and an innersurface 112, constructed using an electrically conductive base, such astitanium, upon which a circuit layer is assembled on the inner surface112 of the device. In one embodiment, the circuit comprises a thin filmceramic insulating layer and a thin film metal layer. In anotherembodiment, the circuit is composed of materials having substantiallysimilar acoustic impedances as surrounding materials residing in theacoustic path between the acoustic energy source and the enclosure. Suchsurrounding materials include patient tissue through which the acousticwaves travel, and elements of the R-S 101 itself.

A plurality of piezoelectric components 120 are affixed to and organizedon the inner surface 112 in which an affixed side/face of thepiezoelectric component 120 forms an aperture 150 with the wall of theenclosure 110. In response to an acoustic field coupling through theaperture 150 to the piezoelectric component 120, the piezoelectriccomponent 120 converts acoustic power to electrical power. The internalspaces 160 between the piezoelectric components 120 contain materials oflow acoustic impedance, such as air, or vacuum making these surfacesnon-isotropic. As shown in FIG. 3A, the inner surface 112 furthercontains a plurality of rectifier circuits 121 each electricallyconnected to a corresponding piezoelectric component 120. Together, apiezoelectric component, an associated aperture and a rectifier functionas a harvesting element.

In one embodiment, the output of each of the harvesting elements isconnected in parallel as illustrated by the abbreviated electricalcircuit in FIG. 6A or connected in series as illustrated by theabbreviated electrical circuit in FIG. 6B, and optionally alsocomprising protection circuitry (not shown) to protect the rectifiersfrom voltages exceeding a threshold. In one embodiment, such a voltagethreshold is approximately 3 to 4 V. In one embodiment, the protectioncircuitry comprises a Zener diode or a series of GaAs Schottky diodes.

In an embodiment where the stimulation electrodes are of a polarizingtype, a direct current (DC) charge can build up on the electrodes unlessa discharge path is provided. In traditional pacemakers, the pacingelectrodes are shorted using a switch to remove any residual charge onthe electrodes when not delivering a stimulus pulse. In embodimentswhere rectifiers are used to convert an alternating current (AC) signalinto a suitable electrical output pulse, there is no discharge path backthrough the rectifier. Therefore, a discharge path may be provided byusing a bleed resistor connected between the stimulation electrodes.Preferably, the bleed resistor is 5-10 times the load impedance in orderto provide a sufficient discharge path while not wasting energy that isintended for delivery to the tissue.

In one embodiment, the R-S 101 comprises circuitry to limit theelectrical output to the stimulation electrodes in order to prevent theelectrical output from exceeding certain currents that would be harmfulto the tissue or would have harmful or undesirable side effects for thepatient.

While the aperture 150 of a harvesting element is determined in part bythe surface area of the piezoelectric component 120 exposed to theacoustic field through the enclosure wall 110, a number of other factorscontribute to the effective acoustic aperture, including piezoelectriccomponent dimensions and materials, mechanical properties of thecoupling surface of the enclosure 110 wall to which the piezoelectriccomponent is affixed, and proximity of neighboring harvesting elements.

In another consideration for the R-S 101 to attain high efficiency inconverting the acoustic field to electrical power, the source impedanceof the combined harvesting elements needs to be matched with the loadimpedance, dictated by the tissue characteristics and the electricalcharacteristics of the stimulation electrodes. This invention allowsmatching the impedance of the R-S 101 (source impedance) to the loadimpedance by a judicious electrical arrangement of the harvestingelements, which could be in series, parallel, or a combination thereof.Another way the source impedance can be manipulated is by changing thedimensions of the individual piezoelectric components 120; for example,changing the cross-section of a piezoelectric component 120 whilekeeping its height constant changes the impedance of that piezoelectriccomponent.

The inner surface 112 may also contain additional circuitry thatconnects the output from the harvesting elements to a pair ofstimulation electrodes, a cathode 113 and an anode 114 (see FIGS. 1C and1D). The electrodes are located on the exterior surface of, or otherwiseattached to or extended from, the R-S 101 device in electrical contactwith the tissue. As described below, in one embodiment, the cathode 113is part of the tissue engagement mechanism located at the tip of the R-S101, and the anode 114 is a portion of or the entire exterior surface ofthe R-S 101. In one embodiment, the R-S 101 device is covered with wovenpolyester (or other similarly suitable material) to promote tissue ingrowth.

The R-S 101 is manufactured by attaching and wire bonding a plurality ofpiezoelectric components 120 and rectifiers 121 to a flat sheet thatbecomes the enclosure 110 by folding or rolling the sheet to produce anR-S “can” assembly. This assembly process is described below andrepresented by FIG. 2 and depicted in FIGS. 3A-3D and in an alternativearrangement of components in FIGS. 3E-3I. As can be appreciated anynumber of arrangements of components can be made. The can assembly 300(shown in FIG. 3 A or alternatively in FIGS. 3E and 3F) is folded toproduce an octagonal (or rolled to produce an otherwise substantiallycylindrical or tubular) can structure 310, as shown in FIGS. 3C and 3Dor alternatively in FIGS. 3G-3I. This process will now be described inmore detail.

FIG. 2 is a flow diagram illustrating a method for manufacturing an R-S101, in accordance with embodiments of the present invention. At step201, a sheet 301 (see FIG. 3A) is masked to avoid depositing insulationor circuit material onto edges that will later be welded. In oneembodiment, the sheet 301 is made of thin titanium. Alternatively, it iscontemplated that the sheet 301 may be made of biocompatible materialsuch as low carbon stainless steel, polyimide, polyester, liquid crystalpolymer (LCP), or the like, or non-biocompatible material coated with abiocompatible material such as those mentioned above.

Optionally, many sheets 301 can be fabricated out of a larger sheet. Insuch an embodiment, all the steps described below up to the folding upof the can assembly can be performed on the larger sheet, after whichindividual can assemblies 300 can be cut from the larger sheet andfolded individually.

At step 202, the sheet 301 is coated with a thin electrical insulationlayer, such as a very thin ceramic layer. Alternatively, anon-conductive polymer layer such as polyimide can be used instead, or aflexible circuit can be laminated on the sheet 301 to achieve the sameresults.

At step 203, the mask is removed, and at step 204 the sheet 301 ismasked again, this time to form a circuit pattern. At step 205, themasked sheet 301 is coated with a conductive material. It iscontemplated that gold or gold over nickel or other such conductivematerials will work particularly well, due to their high conductivityand suitability for wire bonding.

At step 206, the mask is removed to reveal the circuit pattern. As shownin FIG. 3 A, the circuit pattern comprises positive and negativeconductive traces 302 and 303 which will be connected to the positiveand negative outputs of the rectifiers 121. Additional layers ofinsulator and conductor could be added to form more complex circuitrysimilar to conventional circuit boards.

At step 207, the sheet 301 is masked to create half depth fold lines byetching. The masking and the fold lines are preferably created on theouter surface 111 of the sheet 301 to avoid interfering with the circuiton the inner surface 112. However, the masking and fold lines can alsobe created on the inner surface 112 (circuit side), for example, suchthat they do not interfere with the circuit. Alternatively, circuittraces may be created across the fold lines. Alternatively, therectifiers could be attached with flip chip connections eliminating asubstantial fraction of the wire bonds and simplifying routing.

Additionally, half depth outlines may be etched on either outer or innersurfaces that define locations for the piezoelectric components 120.These outlines also contribute to defining the aperture 150 for apiezoelectric component 120, thereby providing mechanical interruptionor isolation, as well as defining an effective aperture which may belarger than the widths of the piezoelectric components 120. Suchacoustic apertures 150 may be defined in a number of alternative ways,for example, by etching away (a) surface material that lies outside ofthe aperture boundaries and leaving material inside of the apertureboundaries unetched, (b) surface material that lies inside of theaperture boundaries and leaving material outside of the apertureboundaries unetched, (c) material at the aperture boundaries (of acertain boundary width) and leaving material inside and outside of theboundaries unetched, or (d) by leaving the aperture boundaries (or acertain boundary width) unetched and etching surface material that liesinside and outside of the aperture boundaries. While such apertures maybe etched on the inner surface 112 or the outer surface Ill of the sheet301, they are preferably etched on the outer surface 111, therebyleaving a smooth surface on the inner surface 112 for attaching thepiezoelectric components 120. Alternatively, the surface may be modifiedby a process other than etching, as known to those of ordinary skill inthe art. The masking and etching of fold lines and aperture geometrycould also be done before plating with insulator and circuitry layers.Acoustic apertures could also be created by attaching other materialssuch as frames or cylinders of titanium, ceramic or other material tothe substrate in the space not occupied by the piezoelectric components.

At step 208 the inner surface of the sheet 301 is masked to also createoutlines of individual can assemblies 300. At step 209, the sheet 301 ischemically etched to create fold lines 304 (as shown in FIG. 3A) andoutlines of individual cans. At step 210, the masks are removed, and atstep 211 the piezoelectric components 120 and rectifiers 121 are mountedonto the sheet 301. In one embodiment, they are mounted using anadhesive. The bottom electrode coating on the piezoelectric componentsmakes electrical contact with the circuit either by intimate mechanicalcontact at high points when the adhesive is thin enough, by using anelectrically conductive adhesive, by reflowing of indium or tin leadalloys to make the electrical and mechanical connection (similar toconventional surface mounting practice), or by using similar techniquesknown to those of ordinary skill in the art.

At step 212, the piezoelectric components 120 are wire bonded to theirrespective rectifiers 121, and the positive and negative outputs of therectifiers 121 are wire bonded to the positive and negative conductivetraces 302 and 303 on the sheet 301. While wire bonding works well dueto the minimal influence it has on the resonant structure, otherattachment methods, such as tab bonding, flip chip connections, orsurface mount technology (SMT) methods would work for other connectionsmade to the piezoelectric components 120 and/or the rectifiers 121.

FIG. 3B is a side-view of the flat sheet can assembly 300 of FIG. 3A,showing wire bonds 311 from the inputs of the rectifiers 121 to circuitpads on the sheet 301 which are connected to piezoelectric components120, wire bonds 312 from the positive outputs of the rectifiers 121 tothe positive conductive traces 302 (see FIG. 3A), and wire bonds 313from the negative outputs of the rectifiers 121 to the negativeconductive traces 303 (see FIG. 3A). In the particular exemplaryconfiguration shown, the harvesting elements are connected in parallel.However, any combination of circuit configuration may be obtained by thelayout of the circuit mask on the sheet 301, including series orparallel configurations or combinations thereof, as will be obvious toone skilled in the art.

Once the can assembly 300 is complete, it is folded into an octagonalstructure or otherwise folded or rolled to a substantially cylindricalcan structure 310 as dictated by the fold lines, as shown in FIG. 3C.FIG. 3D is a cross-sectional view of the folded can structure 310 ofFIG. 3C showing a compact and an optimal arrangement of thepiezoelectric components and the rectifiers. This spatial arrangementallows packing a high number of piezoelectric components into a givenvolume. The fold lines act to isolate the folding strains away from theattachment points of the piezoelectric components 120 and rectifiers121. In one embodiment, the can assembly 300 comprises an interlockingjoint structure 320 (also known as a “dado joint”), as shown in FIGS.3A, 3E, 3F, and 3G. This interlocking joint structure 320 helps alignthe edges of the folded can structure 301 as they are brought togetherthrough folding before the edges are welded together. This allows formaximizing the amount of material immediately adjoining the weld,thereby providing for a complete and substantially hole-free seam uponcompletion of welding. FIG. 3G shows the folded can structure 310 andthe interlocking joint structure 320 prior to welding. FIG. 3H shows thewelded can structure 310 with a welding seam 321 where the interlockingjoint structure 320 provided alignment of the can structure edges.

It is an advantageous aspect that the composite structure of the R-S 101can operate with a high degree of isotropy even if the individualharvesting elements have a low isotropy. This is due to the diversity ofaperture orientations of the harvesting elements. For any given spatialorientation of the R-S 101 relative to the acoustic field, there areharvesting elements whose apertures are oriented such that they are ableto harvest a large portion of the acoustic power that is impingent onthem. For example, in an R-S 101 with an octagonal cross-section asshown in FIGS. 3C and 3D, there are four differently oriented harvestingelements, each contributing a potentially different energy output, butin aggregate contributing to a substantially constant overall poweroutput of the R-S 101 as the direction of the incoming acoustic field isvaried.

In one embodiment, the electrical output of the rectifiers 121 is usedto directly stimulate tissue. In an alternative embodiment, the R-S 101further comprises processing circuitry that manipulates the electricaloutput converted by the rectifiers 121 to produce an electrical signalthat stimulates tissue. The processing circuitry manipulates theelectrical output such that it is suitable for the particularstimulation application at hand, such as cardiac pacing, nervestimulation, brain stimulation, voluntary muscle stimulation, painamelioration, or the like. Such manipulation may involve summing orconditioning the electrical signals from the individual rectifiers 121to produce the biologically stimulating electrical output.

As described above, the R-S 101 further comprises at least twoelectrodes to stimulate patient tissue: a cathode 113 and an anode 114.The cathode 113 and anode 114 are electrically connected as illustratedin the abbreviated electrical circuits of FIG. 6A or 6B to negative andpositive outputs of the rectifiers 121 (or of the processing circuitry,in embodiments where such circuitry is used in the R-S 101). Either orboth of the electrodes 113 and 114 may be mounted on the exteriorsurface of, or otherwise attached to the R-S 101, in some instancesforming a portion of the device enclosure.

In the exemplary embodiments shown in FIGS. 1A-1D, the cathode 113 isdisposed on the needle assembly 107 and is routed through a feed-through106 in the axle assembly 104 at the distal tip of the R-S 101. FIG. 1Cshows the tip of the cathode 113 exposed at the distal tip of the axleassembly 104, and the body of the cathode 113 extending through the axleassembly 104 and into the R-S 101 to electrically connect with aflexible conductive coil 115 that is connected to the negative output ofthe rectifiers 121 (the connection between the conductive coil 115 andrectifiers 121 not shown). FIG. 1D shows the flexible conductive coil115 stretched as the axle assembly 104 is pushed axially forward,maintaining an electrical connection between the cathode 113 and thenegative output of the rectifiers 121 or electrical signal processingcircuitry. In embodiments where a processing circuitry is used, theconductive coil 115 is connected to the negative output of theprocessing circuitry. The anode 114 is represented by a portion of thedevice enclosure and extends into the R-S 101 to electrically connectwith the positive output of the rectifiers 121 electrical signalprocessing circuitry (this internal connection is not shown).

According to the present embodiments, the length of the R-S 101 ispreferably about 4-12 mm, more preferably about 6-10 mm, and mostpreferably about 8 mm; the diameter of the R-S 101 is preferably about3-16 French (1.0 to 5.3 mm), more preferably about 5-12 French (1.7 to4.0 mm), and most preferably about 6-8 French (2.0 to 2.7 mm); theoperating frequency of the R-S 101 is preferably about 200 kHz-3 MHz,more preferably about 600 kHz-1.8 MHz, and most preferably about 950kHz-1.2 MHz; and the number of harvesting elements in the R-S 101 ispreferably about 6-200, more preferably about 30-100, and mostpreferably about 40-60.

As described above, the implantable R-S 101 devices of the presentembodiments are also capable of functioning at a high degree ofisotropy. This means that the composite structure of the R-S 101 deviceproduces output electric power that is constant or nearly constant asthe relative orientation of the R-S 101 to the acoustic source isvaried. It is contemplated that the electric power produced by the R-S101 device and delivered to the tissue in proportion to the incidentacoustic intensity impinging on the R-S 101 will be such that theminimum effective area preferably is no more than −6 dB, more preferablyis no more than −3 dB, and most preferably no more than −1 dB from themaximum effective area as the orientation of the receiver-stimulatorvaries relative to that of the acoustic source.

The R-S 101 assembly may comprise one or more piezoelectric components120 in the shape of a cuboid, a post, a cylinder, or a structure with ahexagonal construction or the like, having a pair of transducerelectrodes formed over opposed surfaces thereof. The cuboid is apreferred embodiment, since a structure with a square or rectangularcross-section is easy to manufacture. Additionally, a cuboid shapesatisfies the requirement of being able to pack the most number ofpiezoelectric components 120 into a given volume. In a first exemplaryembodiment, the piezoelectric component 120 may be composed of asingle-crystal or polycrystalline ceramic piezoelectric material. In apreferred mode, the piezoelectric components operate in resonance, andmore preferably in a thickness mode resonance. Also in a preferredembodiment, the natural structural resonance of the R-S 101 body willoverlap the resonance of the transducer. One advantage of usingsingle-crystal piezoelectric material is that the piezoelectriccomponents can be smaller compared to using polycrystalline ceramicpiezoelectric material, due to the lower velocity of sound insingle-crystal piezoelectric materials. When the piezoelectric materialis formed in the shape of a cuboid, the opposed transducer electrodesmay typically be formed over the two opposing square surfaces of thepiezoelectric component, although transducer electrodes over the othersurfaces may also be used.

In a still further embodiment of the implantable R-S 101 of the presentinvention, the R-S 101 comprises a plurality of individual harvestingelements containing piezoelectric components 120, which themselves willtypically have a maximum dimension that is approximately one-halfwavelength of the expected acoustic wave frequency, but the cumulativelateral dimensions of the R-S 101 will preferably be much greater than asingle wavelength.

The harvesting elements 120 in the can structure 310 have apertureswhich are arranged substantially orthogonal to the longitudinal axis ofthe can structure 310. The least favorable direction of the propagationof acoustic energy is when the energy propagates substantially parallelto this longitudinal axis. Therefore, in an optional embodiment, the R-S101 further comprises a plurality of end cap harvesting elements 120which are organized such that their apertures are substantiallyorthogonal to the long axis of the can structure 310. This orthogonalarrangement allows the combined output of the plurality of harvestingelements of the R-S 101 to maintain a more constant output power whenthe acoustic energy propagates substantially parallel to thelongitudinal axis of the can structure 310.

FIG. 4A is a cross-sectional illustration of an exemplary embodiment ofan axle assembly 104 with end caps. The axle assembly 104 has an end cap501 at the proximal end and another end cap 501 at the distal end. Theend caps 501 comprise harvesting elements whose apertures 150 arearranged orthogonal to the apertures of harvesting elements 120 on thesides of the can structure 310 described above. The can structure 310 isnot shown in FIG. 4 A in order to more clearly illustrate the end caps501. The can structure 310 would be placed between the end caps 501.

FIG. 5A is a flow diagram illustrating a method for assembling an endcap 501, in accordance with an embodiment of the present invention. Atstep 601, an end cap flex circuit 510 is folded and bonded into an endcap structure. FIG. 4B shows an exemplary end cap flex circuit 510. Atstep 603, a plurality of piezoelectric components 120 and rectifiers 121are bonded to the flex circuit 510, as shown in FIG. 4C and in thecross-section view of FIG. 4D. At step 605, the piezoelectric components120 are wire bonded to their respective rectifiers 121, and the positiveand negative outputs of the rectifiers 121 are wire bonded to positiveand negative conductive traces on the end cap flex circuit 510 (similarto the positive and negative conductive traces of the can assembly 300,as described above).

FIG. 5B is a flow diagram illustrating a method for assembling an axleassembly 104, in accordance with an embodiment of the present invention.At step 611, an electrical lead (serving as the cathode 113) is brazedto the internal cathode feed-through 106 of the axle assembly 104. Theaxle assembly 104 is coated with a conductive material at its distaltip, and at step 613 an electrical connection is made from theelectrical lead 113 to the coated axle assembly 104 tip. At step 615,one or more internal tubes are assembled and welded to the internalcathode feed-through 106. The internal tubes complete the hermeticenclosure while allowing a through hole for the axle assembly 104 topass and be deployed. At step 617, the end cap assemblies 501 are weldedto the internal tubes.

In a another aspect of the present invention, methods for transmittingan acoustic field to an implanted R-S 101 comprise implanting an R-S101, typically formed as an assembly of multiple harvesting elements,the R-S 101 having a high degree of isotropy as described above inconnection with the devices of the present invention; directing anacoustic field to the implanted R-S 101 from an acoustic source, whichmay be implanted or located externally, to focus or maximize theacoustic field on the R-S 101; using the harvesting elements that areexposed to the acoustic field through their aperture 150 to transferacoustic power to their associated piezoelectric components 120 which inturn convert the acoustic power to create electrical power; using therectifiers 121 to produce an electrical output from the electrical powerthat is delivered to stimulation electrodes in electrical contact withtissue; and transmitting the acoustic field for sufficient time toproduce sufficient electrical energy to stimulate the tissue. Theelectrical energy flowing between the stimulation electrodes of the R-S101 may possess specific characteristics of voltage, current, waveform,and the like. These electrical characteristics will be selected tostimulate the target cardiac tissue, nerve tissue, brain tissue,voluntary muscle tissue, bone tissue, or the like.

While the above is a complete description of the preferred embodimentsof the invention, various alternatives, modifications, and equivalentsmay be used. Therefore, the above description should not be taken aslimiting the scope of the invention which is defined by the appendedclaims.

What is claimed is:
 1. A method for manufacturing an implantablereceiver-stimulator for converting an acoustic field to electrical powercomprising the steps of: forming an insulating layer on an electricallyconductive sheet; forming a circuit pattern on the insulating layer;electrically connecting a first plurality of acoustic piezoelectriccomponents to the circuit pattern on the sheet; electrically connectinga plurality of individual rectifier circuits to the circuit pattern onthe sheet, the rectifier circuits configured to convert electrical poweroutput from the piezoelectric components to a biologically stimulatingelectrical output; electrically connecting the biologically stimulatingelectrical output to a pair of stimulation electrodes; and folding thesheet into an octagonal or otherwise substantially cylindrical canstructure, thereby arranging the acoustic piezoelectric components suchthat acoustic energy is harvested efficiently from any direction of thepropagating acoustic field.
 2. The method of claim 1, wherein theacoustic piezoelectric components are composed of a piezoelectricmaterial, optionally wherein the piezoelectric material is one of a polycrystalline ceramic piezoelectric material or a single crystalpiezoelectric material.
 3. The method of claim 2, wherein thepiezoelectric components are in the shape of a cuboid, wherein one ofthe faces of the cuboid is mounted to the circuit pattern on the sheet.4. The method of claim 1, wherein the rectifier circuitry comprises oneor more rectifiers connected in parallel and protection circuitry toprotect the rectifiers from damage due to voltages above approximately4V.
 5. The method of claim 4, wherein the protection circuitry comprisesa Zener diode or a series of GaAs Schottky diodes.
 6. The method ofclaim 1, further comprising: affixing an end cap to the can structure,the end cap having a second plurality of acoustic piezoelectriccomponents composed of a piezoelectric material, wherein the secondplurality of piezoelectric components has a piezoelectric axis that issubstantially perpendicular to that of the first plurality ofpiezoelectric components.